The present invention pertains generally to electronic circuits. More specifically, the present invention pertains to electronic circuits which are useful for stabilizing the operation of an amplifier. The present invention is particularly, but not exclusively, useful as a circuit which removes the input bias current from the input potential to an ultrahigh input impedance amplifier in order to stabilize the amplifier""s operation.
It is well known that free space electric fields are emitted from many different types of sources. For example, it is known that organs in the human body, such as the heart and the brain, produce electric fields. Also, electric fields are produced whenever vehicles and machinery are operated. Further, they are emitted from many other diverse types of sources, such as active corrosion sites. In each case, for a variety of different reasons, it may be desirable to detect and measure these electric fields.
At low frequency, antennas that are effective for receiving signals from free space electric fields commonly have the properties of a pure voltage source that is in series with a very small capacitance. Consequently, electric field antennas will typically have an ultrahigh impedance that will often be in a range between ten and one hundred giga ohms (10-100 Gxcexa9). Due to this ultrahigh impedance, any amplifier that may be used to amplify signals of the electric field should have an ultrahigh input impedance.
By definition, xe2x80x9cmatchingxe2x80x9d involves adjusting a load impedance (e.g. amplifier) to the source impedance (e.g. antenna) so that maximum power is received. Stated differently, matching is necessary so that there is minimal reflection loss due to a mismatch between the source and the load. Accordingly, in order for an amplifier (load) to achieve high coupling efficiency and thereby effectively amplify the signals of an electric field (source) it is often necessary for the amplifier to have an ultrahigh input impedance. Additionally, this input impedance must be purely capacitive. The problem here, however, is that any resistive path of even very high resistance value at the amplifier input, will also act as a short to ground. This will then effectively insert an RC filter into the amplifier input circuit that will adversely affect the coupling between the antenna and the amplifier. For example, for an amplifier having an input impedance of 100 Gxcexa9, and a resistive path to ground of 1 Gxcexa9 at the amplifier input, only 1% of the signal will be coupled into the amplifier. Thus, it is generally desirable to remove resistive paths to ground at the amplifier input.
Although the removal of resistive paths from an amplifier input may improve the coupling efficiency of the amplifier, it will not necessarily solve all of the problems that are presented by an ultrahigh input impedance amplifier. Specifically, the removal of resistive paths from the amplifier input still does not account for the amplifier""s inevitable input bias current. In general, the input bias current is a nonsignal current that is either d.c. in nature or has a very low frequency. In any event, unless the input bias current is somehow accounted for, it will flow onto the amplifier input capacitance and onto the source capacitance, and thereby produce a continual drift in the input signal level. This can then lead to saturation of the amplifier. For example, it can be shown that for an input bias current of 50 fA and an input capacitance of 10 pF, an amplifier with a gain of 1000 would saturate in approximately 2 seconds.
In light of the above it is an object of the present invention to provide a system and method for stabilizing an amplifier that enables an ultrahigh input impedance amplifier to be connected directly to a free space electric field antenna without the provision of a current path to ground to shunt the input bias current away from the amplifier input. Another object of the present invention is to provide a system and method for stabilizing an amplifier that allows the amplifier to be connected to ultrahigh impedance sources with high coupling efficiency. Still another object of the present invention is to provide a system and method for stabilizing an amplifier that is effectively easy to use, relatively simple to manufacture, and comparatively cost effective.
A system for stabilizing an ultrahigh input impedance amplifier includes an antenna for receiving an a.c. input signal from a free space electric field. The system also includes an input line that is used to feed the input signal from the antenna to an input port of the amplifier. As envisioned by the present invention, the amplifier will internally generate a substantially d.c. input bias current that combines with the input signal to create an input potential at the input port of the amplifier. As further envisioned by the present invention, the amplifier will also have a guard, and the same input potential will be established on the guard as well as at the input port. Recall, this input potential includes both the input signal and the input bias current. Importantly, the ultrahigh impedance amplifier does not, however, include any uncompensated resistive paths to ground at its input.
In overview, for the operation of the system of the present invention, the input potential is taken from the guard. The input bias current in this input potential is then separated from the input signal to establish a corrective signal. Next, the corrective signal is used to alter the input potential at the input port of the amplifier. The result is that the input bias current is effectively eliminated from the amplifier input. Consequently, the ultrahigh impedance of the antenna can be matched with the ultrahigh impedance of the amplifier, and the amplifier""s operation can be stabilized.
In detail, for one embodiment of the present invention, a capacitor is connected to the guard of the amplifier. Specifically, in this embodiment the capacitor is used to remove the input bias current (essentially a d.c. signal) from the input potential. Thus, the input signal itself (an a.c. signal) is established as the corrective signal. Additionally, a high impedance path is connected between the capacitor and the amplifier""s input port to influence the input potential at the input port with the corrective signal. More specifically, the corrective signal (i.e. input signal) from the capacitor blocks the input signal portion of the input potential at the diode to remove the input bias current from the input potential. The input bias current, by itself, is then provided a path to ground.
For an alternate embodiment of the present invention, a differential amplifier is contemplated. In this case, the system includes a low pass filter that is connected between the guard and an additional input port of the differential amplifier. The low pass filter then blocks transmission of the input signal (essentially a high frequency a.c. signal) in the input potential from the guard. Thus, the input bias current (a low frequency of d.c. signal) is used as the corrective signal. This corrective signal is then fed into the additional input port of the amplifier and is used in the amplifier to cancel the input bias current from the input potential at the input port.
In the specific case where a differential amplifier is used, a combination of the embodiments mentioned above can be incorporated. Specifically, the input potential at the guard can be fed through a low pass filter to the additional input port of the amplifier. As discussed above, this will allow the input bias current to be used for canceling itself in the amplifier. At the same time, the input potential at the guard can be passed through a capacitor to a high impedance path that is connected between the capacitor and the input port of the amplifier. As discussed above, this circuitry allows the input bias current in the input potential to be shunted to ground and to thereby remove the input bias current from the input potential.